REPORT ON THE SIPHONOPHOR^E. 85 



parallel to the axis of vision), the vertical sail runs from our proximal and right to our 

 distal and left side (" Velella sinistra," Chamisso). 



Limbus Umbrella} (fig. 1). — The broad soft border of the horizontal disc is widely 

 prominent over the edge of the pneumatophore, and has four deep marginal incisions. 

 Two opposite ones of these correspond to the diagonal axis of the sail (the macrodia- 

 gonal), and touch the two ends of its base ; the two others correspond to the diagonal 

 axis of the transverse or frontal groove (the brachydiagonal). The border of the mantle 

 is divided by these four marginal incisions into four large lobes, which are disposed in 

 pairs. The two lateral lobes (right and left) are nearly rectangular, and much smaller 

 than the two crescentic sagittal lobes (anterior and posterior). Besides these four deep 

 marginal incisious, which correspond to those of the edge of the pneumatophorous disc, 

 the margin of the mantle-border often exhibits four smaller incisions alternating with 

 the former ; but these are not constant. 



Subumbrella (fig. 2). — The inferior or basal face of the umbrella exhibits the smooth 

 and lobed mantle-border to the same extent as the upper face. A deep elliptical 

 incision, the tentacular or submarginal furrow (ut), separates it from the elliptical central 

 area of the subumbrella. This area is bisected by the longitudinal axis of the spindle- 

 shaped, long and narrow centradenia and the base of the central siphon. Numerous sexual 

 siphons occupy the greatest part of that subumbrellar central area, whilst a double or 

 triple corona of tentacles surrounds it. 



Margin of the Sail. — The triangular vertical sail is composed of an inner chitinous 

 crest arising from the pneumatocyst, and of a canaliferous plate of the exumbrella, 

 covering the two sides of the former. This plate is prolonged over the free margin of 

 the crest, and borders it as a soft contractile limb, which is the uppermost part of the 

 original longitudinal mantle-fold. 



Pneumatocyst (fig. 5, from above; fig. 6, from below; fig. A,p, in vertical section). — 

 The pneumatocyst is composed, as in all species of Velella and Armenista, of two different 

 parts, the horizontal discoidal float filled with air, or the pneumatodisc, and the vertical 

 triangular crest, which is the inner skeleton-plate of the soft sail, perpendicular to the 

 disc, and placed in its greater diagonal. 



The pneumatodisc, or the horizontal float, is sometimes elliptical or nearly rec- 

 tangular in outline, at other times distinctly S-shaped. It is two and a half times as 

 long as broad ; in the largest specimen 50 to 60 mm. long, 18 to 21 mm. broad. 



The four characteristic marginal incisions, which are described above of the mantle- 

 border, are marked also in the periphery of the pneumatodisc (fig. 5) ; the two notches 

 of the macrodiagonal touch the two ends of the base of the crest, whilst the two notches 

 of the brachydiagonal approach the two ends of the frontal furrow. The four cmadrants 

 of the disc, which are separated by these four diagonal notches, are disposed in pairs 

 opposite ; the two lateral pairs form an oblique triangle with concave base, and their 



